Back-illuminated sensor and a method of manufacturing a sensor

RELATED APPLICATIONS

0001This application claims priority from U.S. Provisional Patent Application 62/686,667, entitled “BACK-ILLUMINATED SENSOR AND A METHOD OF MANUFACTURING A SENSOR”, which was filed on Jun. 18, 2018, and is incorporated by reference herein.

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

0002The present application relates to image sensors suitable for sensing radiation in deep UV (DUV) and vacuum UV (VUV) wavelengths, and to methods for making such image sensors. These sensors are suitable for use in photomask, reticle, or wafer inspection systems and for other applications.

Related Art

0003The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.

0004The integrated circuit industry requires inspection tools with increasingly higher resolution to resolve ever smaller features of integrated circuits, photomasks, reticles, solar cells, charge coupled devices etc., as well as detect defects whose sizes are on the order of, or smaller than, those feature sizes.

0005Inspection systems operating at short wavelengths, e.g. wavelengths shorter than about 250 nm, can provide such resolution in many cases. Specifically, for photomask or reticle inspection, it is desirable to inspect using a wavelength identical, or close, to the wavelength that will be used for lithography, i.e. close to 193.4 nm for current generation lithography and close to 13.5 nm for future EUV lithography, as the phase-shifts of the inspection light caused by the patterns will be identical or very similar to those caused during lithography. For inspecting semiconductor patterned wafers, inspection systems operating over a relatively broad range of wavelengths, such as a wavelength range that includes wavelengths in the near UV, DUV, and/or VUV ranges, can be advantageous because a broad range of wavelengths can reduce the sensitivity to small changes in layer thicknesses or pattern dimensions that can cause large changes in reflectivity at an individual wavelength.

0006In order to detect small defects or particles on photomasks, reticles, and semiconductor wafers, high signal-to-noise ratios are required. High photon flux densities are required to ensure high signal-to-noise ratios when inspecting at high speed because statistical fluctuations in the numbers of photons detected (Poisson noise) is a fundamental limit on the signal-to-noise ratio. In many cases, approximately 100,000 or more photons per pixel are needed. Because inspection systems are typically in use 24 hours per day with only short stoppages, the sensors are exposed to large doses of radiation after only a few months of operation.

0007A photon with a vacuum wavelength of 250 nm has energy of approximately 5 eV. The bandgap of silicon dioxide is about 10 eV. Although it would appear that such wavelength photons cannot be absorbed by silicon dioxide, silicon dioxide as grown on a silicon surface must have some dangling bonds at the interface with the silicon because the silicon dioxide structure cannot perfectly match that of the silicon crystal. Furthermore, because the single dioxide is amorphous, there are likely also some dangling bonds within the material. In practice, there will be a non-negligible density of defects and impurities within the oxide, as well as at the interface to underlying semiconductor, that can absorb photons with DUV wavelengths, particularly those shorter than about 220 nm in wavelength. Furthermore, under high radiation flux density, two high-energy photons may arrive near the same location within a very short time interval (nanoseconds or picoseconds), which can lead to electrons being excited to the conduction band of the silicon dioxide by two absorption events in rapid succession or by two-photon absorption.

0008A further requirement for sensors used for inspection, metrology and related applications is high sensitivity. As explained above, high signal-to-noise ratios are required. If the sensor does not convert a large fraction of the incident photons into signal, then a higher intensity light source would be required in order to maintain the same inspection or measurement speed compared with an inspection or metrology system with a more efficient sensor. A higher intensity light source would expose the instruments optics and the sample being inspected or measured to higher light intensities, possibly causing damage or degradation over time. A higher intensity light source would also be more expensive or, particularly at DUV and VUV wavelengths, may not be available.

0009DUV and VUV wavelengths are strongly absorbed by silicon. Such wavelengths may be mostly absorbed within about 10 nm or a few tens of nm of the surface of the silicon. The efficiency of a sensor operating at DUV or VUV wavelengths depends on how large a fraction of the electrons created by the absorbed photons can be collected before the electrons recombine. Silicon dioxide can form a high-quality interface with silicon with a low density of defects. Most other materials including many of those commonly used for anti-reflection coatings, if deposited directly on silicon, result in a very high density of electrical defects at the surface of silicon. A high density of electrical defects on the surface of silicon may not be an issue for a sensor intended to operate at visible wavelengths, as such wavelengths may typically travel about 100 nm or more into the silicon before being absorbed and may, therefore, be little affected by electrical defects on the silicon surface. However, DUV and VUV wavelengths are absorbed so close to the silicon surface that electrical defects on the surface and/or trapped charged within the layer(s) on the surface can result in a significant fraction of the electrons created recombining at, or near, the silicon surface and being lost, resulting in a low efficiency sensor.

0010U.S. Pat. Nos. 9,496,425 and 9,818,887, both to Chern et al., describe image sensor structures and methods of making image sensors that include a boron layer deposited on, at least, an exposed back surface of the image sensor. Different ranges of temperature for deposition of the boron are disclosed, including a range of about 400-450° C. and a range of about 700-800° C. The inventors have discovered that one advantage of a higher deposition temperature for the boron, such as a deposition temperature between about 600° C. and about 900° C., is that at such temperatures boron diffuses into the silicon providing a very thin, heavily p-type doped silicon layer on the light-sensitive back surface. This p-type doped silicon layer is important for ensuring a high quantum efficiency to DUV and VUV radiation because it creates a static electric field near the surface that accelerates electrons away from the surface into the silicon layer. The p-type silicon also increases the conductivity of the back surface of the silicon, which is important for high-speed operation of an image sensor, since a return path is needed for ground currents induced by the switching of signals on electrodes on the front surface of the sensor.

0011However, processing temperatures higher than 450° C. cannot be used on semiconductor wafers that include conventional CMOS circuits because 450° C. is close to the melting point metals such as aluminum and copper commonly used in fabricating CMOS devices. At high temperatures, such as those greater than 450° C., these metals expand, become soft and can delaminate. Furthermore, at high temperatures copper can easily diffuse through silicon which will modify the electrical properties of the CMOS circuits. Thinning a wafer before any metals are deposited on it allows a boron layer to be deposited on the back surface as described in the aforementioned patents at a temperature between 600 and 900° C. enabling boron to diffuse into the surface during, or subsequent to, the deposition of the boron layer. Subsequently metal interconnects can be formed on the front surface. After the image sensor regions of the wafer have been thinned, for example to a thickness of about 25 μm or thinner, the thinned region can be significantly warped and may have peak-to-valley non-flatness of many tens of microns or more. So, it is necessary to use relatively wide metal interconnect lines and vias, such as multiple microns wide or more, to ensure that the lines and vias connect in spite of any misalignment caused by the non-flatness. Such wide metal interconnects and vias increase the capacitance per unit area associated with those lines and vias. Furthermore, wide interconnects and vias can make it difficult, or impossible, to interconnect all the signals on a large area sensor with about one million or more pixels. In some cases, polysilicon jumpers may be needed to connect together metal interconnects, but polysilicon has much higher resistivity than any metal, so the use of such jumpers can limit the maximum operating speed of a sensor.

0012Therefore, a need arises for an image sensor capable of efficiently detecting high-energy photons without degrading yet overcoming some, or all, of the above disadvantages. In particular, a method of fabricating a back-thinned image sensor with a boron layer and boron doping on its back surface while allowing formation of metal interconnects on a relatively flat wafer (i.e. with a flatness of about 10 μm or less) would allow the use of finer design rules (such as the design rules corresponding to a 0.35 μm process or finer). Such a method would allow narrower metal lines connecting to critical features such as the floating diffusion, enabling smaller floating-diffusion capacitance and higher charge to voltage conversions ratios. Finer design rules also allow more interconnect lines per unit area of the sensor and allow more flexibility in connecting the circuits on the image sensor.

SUMMARY OF THE DISCLOSURE

0013Image sensors and methods of fabricating image sensors with high-quantum-efficiency for imaging DUV and/or VUV are described. These image sensors are capable of long-life operation under high fluxes of DUV and VUV radiation. These methods include process steps to form light sensitive active and/or passive circuit elements in a layer on a semiconductor (preferably silicon) wafer, as well as forming metal interconnections between the electrical elements of the sensor. These image sensors can include fine metal interconnects and vias (such as those conforming to about 0.35 μm, or finer, design rules), while having a backside surface coated with a boron layer and having a highly doped p-type silicon layer immediately adjacent to the boron layer. The metal interconnections may comprise tungsten, molybdenum or other refractory (i.e. high melting point) metal. In one embodiment, the metal interconnections may consist of only refractory metals. In one embodiment, the deposition processes used to form the metal interconnections may be configured to reduce the stress within the metal layers. In one embodiment additional metal interconnections comprising aluminum or copper may be added on top of, and connected to, metal interconnections comprising the refractory metal.

0014An exemplary method of fabricating an image sensor includes forming an epitaxial layer on a substrate, forming a gate layer on the epitaxial layer, the gate layer comprising one or more layers of dielectric materials such as silicon dioxide and silicon nitride, forming circuit elements on the gate layer comprising polysilicon and dielectric materials, forming first metal vias and first metal interconnects to connect together at least some of those circuit elements, thinning the substrate to expose at least a portion of the epitaxial layer (the exposed epitaxial layer is referred to herein as a semiconductor membrane), forming a pure boron layer directly on the exposed portions of the epitaxial layer, diffusing boron into the epitaxial layer during and/or subsequent to forming the boron layer, and optionally forming one, or more, anti-reflection layers directly on the surface of the boron layer. As used herein, the phrase “circuit elements” refers to light sensitive devices such as charge-coupled devices and photodiodes, other semiconductor devices such as transistors, diodes, resistors and capacitors, and electrical interconnections (often called interconnects) between them. These circuit elements are formed using standard semiconductor manufacturing processes including, but not limited to, photolithography, deposition, etching, ion implantation and annealing. The first metal interconnects comprise a refractory metal such as tungsten or molybdenum. Thinning the sample (e.g. a wafer) can be performed using ion etching, chemical etching and/or polishing. Notably, this thinning can increase the sensitivity of the image sensor to light impinging the back surface. An anti-reflection coating may be formed on the boron layer. The method may further include forming one or more additional interconnect layers on top of, and connected to, the first interconnects after forming the pure boron layer. The additional interconnect layers may comprise a metal such as aluminum or copper, as these layers need not be exposed to temperatures greater than about 450° C. The additional interconnect layers may be fabricated following design rules corresponding to a 1 μm process or coarser so that they may be formed on a surface that may have 10 μm or more of non-flatness.

0015Another method of fabricating an image sensor includes forming an epitaxial layer on a substrate, then forming circuit elements on the epitaxial layer. This step includes forming metal interconnects. The metal interconnects may comprise, or may consist entirely of, refractory metals such as tungsten and molybdenum. A protective layer may be formed on the circuit elements. A handle wafer may be bonded to the surface that includes the circuit elements. The substrate is then thinned to expose, at least part of, the epitaxial layer. As indicated above, this thinning can increase the sensitivity of the image sensor to light impinging on the back surface. A pure boron layer is formed on the surface of the epitaxial layer exposed in the thinning process. The pure boron layer may be deposited at a temperature higher than 600° C. or may be raised to a temperature higher than 600° C. after deposition to cause boron to diffuse into the epitaxial layer. An anti-reflection coating may be formed on the boron layer.

0016Image sensors with high-quantum-efficiency and long-life operation for DUV, and/or VUV radiation are described. These image sensors are thinned from the back-side so that they are highly sensitive to radiation impinging on the back-side of the image sensors (wherein these image sensors are back-illuminated). The image sensors include first metal interconnects comprising, or consisting entirely of, refractory metals such as tungsten and molybdenum. Deposited directly on the back surface of the epitaxial layer is a thin (e.g. between about 2 nm and about 20 nm thick) layer high-purity amorphous boron. In one embodiment, one or more additional layers of material may be coated on the boron. The thickness and material of each layer may be chosen to increase the transmission of a wavelength of interest into the image sensor, and/or to protect the boron layer from damage. In one embodiment second metal interconnects may be formed on top of, and connected to, the first metal interconnects. The second metal interconnects may comprise one of aluminum and copper, and may be arranged according to 1 μm or coarser design rules.

0017The image sensors described herein may be fabricated using CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) technology. The image sensors may be two-dimensional area sensors, or one-dimensional array sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

0018<figref idref="DRAWINGS">FIG. 1</figref> is a cross-sectional view showing an exemplary image sensor produced in accordance with the present invention.

0019<figref idref="DRAWINGS">FIGS. 2 and 3</figref> illustrate an exemplary method for fabricating an image sensor.

0020<figref idref="DRAWINGS">FIGS. 4A-4I</figref> illustrate exemplary cross-sections of a wafer subjected to the method described in reference to <figref idref="DRAWINGS">FIGS. 2 and 3</figref>.

0021<figref idref="DRAWINGS">FIG. 5</figref> illustrates an exemplary detector assembly incorporating an image sensor, a silicon interposer, and other electronics.

DETAILED DESCRIPTION OF THE DRAWINGS

0022<figref idref="DRAWINGS">FIG. 1</figref> is a cross-sectional side view depicting a portion of an image sensor <b>100</b> configured to sense deep ultraviolet (DUV) radiation, vacuum ultraviolet (VUV) radiation, extreme ultraviolet (EUV) radiation or charged particles according to an exemplary embodiment of the present invention. Image sensor <b>100</b> includes a circuit element <b>103</b> formed on an upper (first) surface <b>101</b>U of a semiconductor membrane <b>101</b>, first metal interconnects <b>110</b> disposed in dielectric material layers <b>112</b> over circuit element <b>103</b>, and a pure boron layer <b>106</b> formed on a lower (second) surface <b>101</b>L of semiconductor membrane <b>101</b>.

0023In one embodiment, semiconductor membrane <b>101</b> comprises a layer of lightly p-doped epitaxial silicon having a thickness T1 in a range of 10 μm to 40 μm and a p-type (boron) dopant concentration in a range of about 10¹³cm⁻³to 10¹⁴cm⁻³.

0024Circuit element <b>103</b> includes a sensor device (e.g., a light sensitive device such as a photodiode) and associated control transistors that are formed on (i.e., into and over) an upper (first) surface <b>101</b>U of semiconductor membrane <b>101</b>. In the depicted exemplary embodiment, circuit element <b>103</b> includes spaced-apart n+ doped diffusion regions <b>103</b>-<b>11</b>, <b>103</b>-<b>12</b> and <b>103</b>-<b>13</b> that extend from upper surface <b>101</b>U into corresponding portions of semiconductor membrane <b>101</b>, and polycrystalline silicon (polysilicon) gate structures <b>103</b>-<b>21</b> and <b>103</b>-<b>22</b> that are respectively separated from upper surface <b>101</b>U by intervening gate oxide layers. The depicted configuration of the diffusion regions and gate structures forming circuit element <b>103</b> is provided solely to for purposes of describing exemplary circuit element structures and is not intended to represent a functional sensor device or to limit the appended claims.

0025In one embodiment, pure boron layer <b>106</b> is formed using techniques described below such that pure boron layer <b>106</b> has a thickness T2 in the range of 2 nm and 20 nm. In one embodiment, pure boron layer <b>106</b> comprises a boron concentration of 80% or higher, with inter-diffused silicon atoms and oxygen atoms predominantly making up the remaining 20% or less.

0026According to a first aspect, image sensor <b>100</b> includes a heavily p-doped region <b>102</b> that extends from lower surface <b>101</b>L into semiconductor membrane <b>101</b> (i.e., such that p-doped region <b>102</b> is disposed immediately adjacent to pure boron layer <b>106</b>). In one embodiment, p-doped region <b>102</b> is a boron-doped region that is formed by the diffusion of boron atoms through lower surface <b>101</b>L during, or immediately following, the formation of pure boron layer <b>106</b>. Preferably p-doped region <b>102</b> should have a maximum dopant concentration immediately adjacent to lower surface <b>101</b>L, with the dopant concentration decreasing with distance into semiconductor membrane <b>101</b> away from lower surface <b>101</b>L. In an exemplary embodiment, p-doped region <b>102</b> has a nominal p-type doping concentration greater than 10¹⁹cm⁻³immediately adjacent to lower surface <b>101</b>L and decreases to a dopant concentration similar to that of the semiconductor membrane <b>101</b> (for example a dopant concentration in the range of 10¹³cm⁻³to 10¹⁴cm⁻³) over a distance of between 10 nm and 50 nm away from that lower surface.

0027According to a second aspect, first metal interconnects <b>110</b> and corresponding first metal vias <b>115</b> comprise a refractory metal (e.g., one or both of tungsten and molybdenum) for reasons that will become clear below. In the exemplary embodiment, first metal interconnects include metal lines <b>110</b>-<b>1</b>, <b>110</b>-<b>2</b> and <b>110</b>-<b>3</b> that are respectively formed as part of three metallization processes M<b>1</b>, M<b>2</b> and M<b>3</b>, which also include the formation of first metal vias <b>115</b> such that first metal interconnects <b>110</b> are electrically connected to associated portions of circuit element <b>103</b>. The configuration of first metal interconnects <b>110</b> and first metal vias <b>115</b> is arbitrarily depicted for illustrative purposes and not intended to be limiting (e.g., first metal interconnects <b>110</b> may be formed in any number of metal layers).

0028In another specific embodiment, an optional anti-reflection coating <b>108</b> is deposited on a lower (outward-facing) surface <b>106</b>L of pure boron layer <b>106</b>, and thickness T2 of pure boron layer <b>106</b> is in the range of 3 nm to 10 nm.

0029In one embodiment, image sensor <b>100</b> includes an optional protection layer <b>104</b> that is formed over dielectric layer <b>112</b> such that (first) metal interconnects <b>110</b> are entirely disposed between protection layer <b>104</b> and semiconductor membrane <b>101</b>. In alternative embodiments, protection layer <b>104</b> is either implemented by a portion of a handler wafer (e.g., monocrystalline silicon or glass) or implemented by a layer of protective material (e.g., silicon dioxide, silicon nitride or silicon oxynitride) that is attached/formed over dielectric layer <b>112</b>. In one embodiment, protection layer <b>104</b> has a thickness in the range of 0.5 μm to 3 μm. In other embodiments, protection layer <b>104</b> is omitted entirely.

0030In one embodiment, one or more second metal interconnects <b>120</b> are formed in a second dielectric layer <b>122</b> over first metal interconnects <b>110</b> and coupled to circuit element <b>103</b> by way of second metal vias. For example, second metal interconnect <b>120</b>-<b>1</b> is connected directly to a portion of circuit element <b>103</b> by way of second metal via <b>125</b>-<b>1</b>, and second metal interconnect <b>120</b>-<b>2</b> is electrically connected to circuit element <b>103</b> by way of second metal via <b>125</b>-<b>2</b> and first metal interconnect <b>110</b>-<b>2</b>. In one embodiment, second metal interconnects <b>120</b> comprise at least one of aluminum and copper. In one embodiment, second metal interconnects <b>120</b> are disposed over protection layer <b>104</b>, and second metal vias <b>125</b>-<b>1</b> and <b>125</b>-<b>2</b> extend through protection layer <b>104</b>.

0031<figref idref="DRAWINGS">FIGS. 2 and 3</figref> illustrate an exemplary technique <b>200</b> for fabricating an image sensor. In this embodiment, the circuit elements can be created in step <b>201</b> using standard semiconductor processing steps including lithography, deposition, ion implantation, annealing, and etching. In one embodiment, CCD and/or CMOS sensor elements and devices may also be created in step <b>201</b>. These circuit elements are created in an epitaxial (epi) layer on the front-side surface of the wafer. In preferred embodiments, the epitaxial layer is about 10 μm to 40 μm thick. The epitaxial layer is lightly p doped (p-). In one embodiment, the epitaxial layer resistivity is between about 10 and 1000 Ωcm. First metal interconnects are created in step <b>201</b> using tungsten, molybdenum or other refractory metal, so that the metal interconnects can withstand high temperatures (such as temperatures greater than about 600° C.) in subsequent steps, in particular, in steps <b>209</b> and/or <b>211</b>. Preferably the composition of the first metal interconnects and the processes used to deposit those interconnects may be chosen to reduce the stress in the metals. Deposition processes for depositing reduced stress tungsten are described by, for example, Schmitz et al. in “The Dependence of the Stress of Chemical Vapor Deposited Tungsten Films on Deposition Parameters”, J. Electrochem. Soc., 141, pp 843-848 (1994). This document is incorporated herein by reference.

0032In step <b>203</b>, the front-side surface of the wafer can be protected. This protection may include depositing one or more protective layers on top of the circuit elements formed during step <b>201</b>. The one or more protective layers may comprise silicon dioxide, silicon nitride or other material. This protection may also, or instead, include attaching the wafer to a handling wafer, such as a silicon wafer, a quartz wafer, or a wafer made of other material. The handling wafer may include vias comprising a refractory metal such as tungsten or molybdenum for connecting to the circuit elements.

0033Step <b>205</b> involves thinning the wafer from the back-side so as to expose the epitaxial layer in, at least, the active sensor areas. This step may involve polishing, etching, or both. In some embodiments, the entire wafer is back-thinned. In other embodiments, only the active sensor areas are thinned all the way to the epitaxial layer.

0034Step <b>207</b> includes cleaning and preparing the back-side surface prior to the boron deposition. During this cleaning, the native oxide and any contaminants, including organics and metals, should be removed from the back-side surface. In one embodiment, this cleaning can be performed using a dilute HF solution or using an RCA clean process. After cleaning and during preparation, the wafer can be dried using the Marangoni drying technique or a similar technique to leave the surface dry and free of water marks.

0035In preferred embodiments, the wafer is protected in a controlled atmosphere between steps <b>207</b> and <b>209</b> (using, e.g. dry nitrogen) to minimize native oxide regrowth after the cleaning.

0036In step <b>209</b>, boron is deposited on the back-side surface of the wafer. In one preferred embodiment, this deposition can be done using a mixture of diborane and hydrogen gases at a temperature of about 600-900° C., thereby creating a high-purity amorphous boron layer. In an alternative embodiment, the deposition may be done using diborane, or a diborane-hydrogen mixture, diluted in nitrogen. The thickness of the deposited boron layer depends on the intended application for the sensor. Typically, the boron layer thickness will be between about 2 nm and 20 nm, preferably between about 3 nm and 10 nm. The minimum thickness is set by the need for a pinhole-free uniform film, whereas the maximum thickness depends on the absorption of the photons or charged particles of interest by the boron, as well as the maximum length of time that the wafer can be kept at the elevated temperature.

0037In step <b>209</b>, the wafer can be held at a high-temperature for a few minutes in a reducing environment, such as a hydrogen gas. In preferred embodiments, the wafer can be held at a temperature of approximately 800° C. to 850° C. for about 1 to 4 minutes. This high temperature can remove any native oxide layer that might have regrown following step <b>207</b>.

0038More details on boron deposition can be found in “Chemical vapor deposition of a-boron layers on silicon for controlled nanometer-deep p⁺-n junction formation,” Sarubbi et al., J. Electron. Material, vol. 39, pp. 162-173, 2010, which is incorporated by reference herein.

0039In step <b>211</b>, the wafer is held at a high temperature, such as a temperature between about 600° C. and about 900° C. for a few minutes, such as for a time between about 1 and 10 minutes, to allow boron to diffuse into the surface of the epitaxial layer (e.g., thereby forming boron doped region (doped layer) <b>102</b> in membrane <b>101</b> just inside surface <b>101</b>L). The temperature used in step <b>211</b> may be similar to, or higher than, the temperature used in <b>209</b> for depositing the boron layer. Preferably step <b>211</b> is performed in an environment of nitrogen, argon or other inert gas.

0040After step <b>211</b>, other layers may be deposited on top of the boron layer. These other layers may include anti-reflection coatings comprised of one or more materials, such as silicon dioxide, silicon nitride, aluminum oxide, hafnium dioxide, magnesium fluoride, and lithium fluoride. These other layers may include a thin protective layer comprising a metal such as ruthenium, tungsten or molybdenum. One or more of these other layers may be deposited using (atomic layer deposition) ALD. An advantage of using an ALD process for depositing these layers is that ALD processes typically allow very precise (single monolayer) control of the thickness of the deposited layer(s). In an alternative embodiment, other layers may be deposited on top of the boron layer after one of the later process steps, such as one of the steps depicted in <figref idref="DRAWINGS">FIG. 3</figref>.

0041In one embodiment, the protective front-side layer and/or handling wafer may be partially or entirely removed in step <b>213</b> in order to expose at least some circuit elements and/or first metal interconnects.

0042In one embodiment, second metal interconnects may be fabricated on the front-side of the wafer and connected to the first metal interconnects and/or circuit elements in step <b>215</b>. Second metal interconnects may comprise any convenient metal including copper or aluminum since second metal interconnects may not be subjected to high temperatures (such as temperatures exceeding 450° C.) during subsequent processing steps. Second metal interconnects may be deposited by any appropriate processing technique including, but not limited to, electro-plating, electroless plating, chemical vapor deposition (CVD), ALD, or physical vapor deposition (PVD). Since the wafer may be less flat at step <b>215</b> than in step <b>201</b>, the patterns of the second metal interconnects may follow relatively large design rules (such as design rules corresponding to a 1 μm or coarser process) and may be printed by mask aligner, contact mask or other relatively low-resolution lithography process. The second metal interconnects can form a conduction path in parallel with selected first metal interconnects, thereby reducing the overall resistance of those interconnects and enabling higher-speed operation of the image sensor than would otherwise be possible by first metal interconnects alone, as the first metal interconnects may have relatively higher resistance due to the use of a refractory metal.

0043In step <b>223</b> external connections, such as bonding pads or bump pads are fabricated on the wafer and electrically connected to the circuit elements, for example by connecting to first metal interconnects or to second metal interconnects. The external connections formed in step <b>223</b> may be on the front side, back side or both sides of the wafer. In one embodiment, connecting the external connections to first metal interconnects or second metal interconnects may involve creating, opening or exposing vias in the handling wafer, protective front-side layer, or the wafer. In one embodiment through-silicon vias are used to connect external connections on the back side of the wafer to first metal interconnects or second metal interconnects on the front side of the wafer.

0044In step <b>225</b>, the resulting structure may be packed in a suitable package. The packing step may comprise flip-chip bonding or wire bonding of the device to the substrate. The package may include a window that transmits wavelengths of interest, or it may comprise a flange or seal for interface to a vacuum seal.

0045<figref idref="DRAWINGS">FIGS. 4A-4I</figref> illustrate exemplary cross-sections of a wafer subjected to method <b>200</b> (<figref idref="DRAWINGS">FIGS. 2 and 3</figref>). <figref idref="DRAWINGS">FIG. 4A</figref> illustrates an epitaxial (epi) layer <b>402</b> formed on the front side of a substrate <b>401</b>. Epi layer <b>402</b> is preferably a p-epi layer. In one embodiment, the epi layer resistivity is between about 10 and 1000 Ωcm.

0046<figref idref="DRAWINGS">FIG. 4B</figref> illustrates various circuit elements <b>403</b> including first metal interconnects formed on the epi layer (step <b>201</b>). Because first metal interconnects <b>410</b> are formed while substrate <b>401</b> is still hundreds of microns thick and hence not severely warped, first metal interconnects <b>410</b> can be formed using normal sub-micron CMOS processing techniques and may include multiple layers of high-density metal interconnects. First metal interconnects <b>410</b> comprise a refractory metal such as tungsten or molybdenum. In one embodiment, first metal interconnects <b>410</b> consist entirely of refractory metals. In one embodiment, multiple through-silicon vias (TSV) <b>403</b>A are created around one, or more, edges of the image sensor array in order to allow connection to the circuit elements <b>403</b>.

0047<figref idref="DRAWINGS">FIG. 4C</figref> illustrates a supporting or handling wafer <b>404</b> attached to the top of the circuit elements <b>403</b> (step <b>203</b>). Note that the through-silicon vias are shown, but not labeled so as not to overly complicate the drawings. In an alternative embodiment, a protective layer can be used instead of, or in addition to, supporting or handling wafer <b>404</b>. In one embodiment (not shown), vias are formed in wafer or layer <b>404</b> to allow connection to the circuit elements <b>403</b>.

0048<figref idref="DRAWINGS">FIG. 4D</figref> illustrates the wafer after substrate <b>401</b> is back-thinned (step <b>205</b>) to expose some or all of the back-side surface of epi layer <b>402</b> (i.e., the surface opposite to which circuit elements <b>403</b> are formed). In the case where substrate <b>401</b> is entirely removed, epi layer <b>402</b> forms the semiconductor membrane. As depicted in <figref idref="DRAWINGS">FIG. 4D</figref>, a native oxide <b>402</b>A may form on the surface of epi layer <b>402</b> exposed by the back thinning.

0049<figref idref="DRAWINGS">FIG. 4E</figref> illustrates the wafer after a cleaning and preparation of the back-side surface (step <b>207</b>).

0050<figref idref="DRAWINGS">FIG. 4F</figref> illustrates a pure boron layer <b>406</b> after being formed on the back-side surface of the epi layer <b>402</b> (step <b>209</b>). Diffusion of the boron into the epi layer creates a thin (a few nanometers to a few tens of nanometers) heavily p-doped silicon layer (not shown) at the surface of the epi layer adjacent to the boron layer (step <b>211</b>).

0051<figref idref="DRAWINGS">FIG. 4G</figref> illustrates one or more optional anti-reflection or protection layers <b>408</b> deposited on top of the pure boron layer <b>406</b>. At least one of the layers may be deposited using an ALD process.

0052<figref idref="DRAWINGS">FIG. 4H</figref> illustrates the wafer in one embodiment where protective layer or support or handling wafer <b>404</b> has been fully or partially removed (step <b>213</b>) and second metal interconnects (<b>409</b>) fabricated on the front side (step <b>215</b>). As explained above in reference to <figref idref="DRAWINGS">FIGS. 2 and 3</figref>, second metal interconnects <b>409</b> may comprise a metal such as aluminum or copper and may be patterned following much coarser design rules than those used for the first metal interconnects included in circuit elements <b>403</b>.

0053<figref idref="DRAWINGS">FIG. 4I</figref> illustrates the wafer after fabrication of metal pads <b>407</b><i>a </i>and <b>407</b><i>b </i>(step <b>223</b>) by appropriate patterning, etching and deposition steps. Pad <b>407</b><i>a </i>is fabricated on the front side and electrically connects, for example, to second metal interconnects <b>409</b>. Pad <b>407</b><i>b </i>is fabricated on the back side and electrically connects to TSV <b>403</b>A. As explained above in reference to <figref idref="DRAWINGS">FIGS. 2 and 3</figref>, the image sensor may include pads only on the front surface (such as pad <b>407</b><i>a</i>), only on the back surface (such as pad <b>407</b><i>b</i>) or on both front and back surfaces. Note that if optional second metal interconnects are not present, the metal pads will be directly electrically connected to the circuit elements <b>403</b>.

0054The above examples are not meant to limit the scope of the invention disclosed herein. They are meant merely as illustrations of how first metal interconnects comprising, or consisting of, refractory metals can be used to fabricate an image sensor, which is subsequently coated with a boron layer on its photo-sensitive surface. Because the first metal interconnects comprise refractory metals, they can withstand high temperatures during the boron deposition and diffusion.

0055<figref idref="DRAWINGS">FIG. 5</figref> illustrates an exemplary detector assembly <b>500</b> incorporating an image sensor <b>504</b>, a silicon interposer <b>502</b> and other electronics in accordance with certain embodiments of the present invention.

0056In one aspect of the present invention, the detector assembly <b>500</b> may include one or more light sensitive sensors <b>504</b> disposed on the surface of an interposer <b>502</b>. In one embodiment, the one or more interposers <b>502</b> of the assembly <b>500</b> may include, but are not limited to, a silicon interposer. In a further aspect of the present invention, the one or more light sensitive sensors <b>504</b> of the assembly <b>500</b> are back-thinned and further configured for back-illumination including a boron layer and a highly doped layer adjacent to the boron layer as described herein.

0057In another aspect of the present invention, various circuit elements of the assembly <b>500</b> may be disposed on or built into the interposer <b>502</b>. In one embodiment, one or more amplification circuits (e.g., charge conversion amplifier) (not shown) may be disposed on or built into the interposer <b>502</b>. In another embodiment, one or more conversion circuits <b>508</b> (e.g., analog-to-digital conversion circuits, i.e. digitizers <b>508</b>) may be disposed on or built into the interposer <b>502</b>. In another embodiment, one or more driver circuits <b>506</b> may be disposed on or built into the interposer <b>502</b>. For example, the one or more driver circuits <b>506</b> may include a timing/serial drive circuit. For instance, the one or more driver circuits <b>506</b> may include, but are not limited to, clock driver circuitry or reset driver circuitry. In another embodiment, one or more decoupling capacitors (not shown) may be disposed on or built into the interposer <b>502</b>. In a further embodiment, one or more serial transmitters (not shown in <figref idref="DRAWINGS">FIG. 5</figref>) may be disposed on or built into the interposer <b>502</b>.

0058In another aspect of the present invention, one or more support structures may be disposed between the bottom surface of the light sensitive array sensor <b>504</b> and the top surface of the interposer <b>502</b> in order to provide physical support to the sensor <b>504</b>. In one embodiment, a plurality of solder balls <b>516</b> may be disposed between the bottom surface of the light sensitive array sensor <b>504</b> and the top surface of the interposer <b>502</b> in order to provide physical support to the sensor <b>504</b>. It is recognized herein that while the imaging region of the sensor <b>504</b> might not include external electrical connections, the back-thinning of the sensor <b>504</b> causes the sensor <b>504</b> to become increasingly flexible. As such, solder balls <b>516</b> may be utilized to connect the sensor <b>504</b> to the interposer <b>502</b> in a manner that reinforces the imaging portion of the sensor <b>504</b>. In an alternative embodiment, an underfill material may be disposed between the bottom surface of the light sensitive array sensor <b>504</b> and the top surface of the interposer <b>502</b> in order to provide physical support to the sensor <b>504</b>. For example, an epoxy resin may be disposed between the bottom surface of the light sensitive array sensor <b>504</b> and the top surface of the interposer <b>502</b>.

0059In another aspect of the present invention, the interposer <b>502</b> and the various additional circuitry (e.g., amplification circuit, driver circuits <b>506</b>, digitizer circuits <b>508</b>, and the like) are disposed on a surface of a substrate <b>510</b>. In a further aspect, the substrate <b>510</b> includes a substrate having high thermal conductivity (e.g., ceramic substrate). In this regard, the substrate <b>510</b> is configured to provide physical support to the sensor <b>504</b>/interposer <b>502</b> assembly, while also providing a means for the assembly <b>500</b> to efficiently conduct heat away from the imaging sensor <b>504</b> and the various other circuitry (e.g., driver circuitry <b>506</b>, digitizer <b>508</b>, amplifier, and the like). It is recognized herein that the substrate may include any rigid highly heat conductive substrate material known in the art. For example, the substrate <b>510</b> may include, but is not limited to, a ceramic substrate. For instance, the substrate <b>510</b> may include, but is not limited to, aluminum nitride.

0060In another embodiment, the substrate <b>510</b> may be configured to provide an interface to a socket or an underlying printed circuit board (PCB). For example, as shown in <figref idref="DRAWINGS">FIG. 5</figref>, the substrate <b>510</b> may provide interconnection between the interposer <b>502</b> and a socket or a PCB via interconnects <b>512</b>. Those skilled in the art will recognize that the substrate <b>510</b> may be operatively coupled to an underlying PCB and further electrically coupled to a socket or PCB in a variety of ways, all of which are interpreted to be within the scope of the present invention.

0061The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the particular embodiments described. For example, additional steps may be added to the flow chart depicted in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>, or some of the steps shown may be done in different sequence than shown. Thus, the invention is limited only by the following claims and their equivalents.